1. Field of the Invention
The invention relates to a lens device, more particularly to a lens device for image magnification of a specimen.
2. Description of the Related Art
Due to sophistication in technology advancement, many counterfeits, such as paper money, checks, etc., are forged to such a fine precision that makes them difficult to be readily detected as a forgery. If, without any proper countermeasure, these counterfeits can be unnoticeably circulated in a market, they can create considerable losses for both consumers and merchants.
U.S. Pat. No. 7,102,817 entitled “Objective Lens Device for a Microscope” discloses a lens device that can be used to inspect suspected counterfeits. Referring to FIGS. 1 and 2, the conventional lens device includes a casing 10, a telescopic tube assembly 11, and a cap 12. The telescopic tube assembly 11 has an outer tube 111 that is sleeved rotatably in the casing 10, an inner tube 112 mounted in and coupled threadedly to the outer tube 111, and movable in a longitudinal direction of the outer tube 111, and an objective lens 113 mounted in the inner tube 112 and through which a specimen 100 can be observed. The cap 12 is disposed on the casing 10 and includes an image sensor 121 for capturing images of the specimen 100 from the objective lens 113, and a pair of spaced-apart guiding rods 122 that extend through the inner tube 112.
When adjusting a focal length between the objective lens 113 and the specimen 100, the outer tube 111 is rotated, and the inner tube 112 moves longitudinally in the outer tube 111 under the guidance of the guiding rods 122.
The image sensor 121 is electrically coupled to an external image viewer (i.e., a computer). However, in the above-described arrangement, the lens device is not provided with any light source for illumination of the specimen 100. Light plays an important role for obtaining a better image of the specimen 100. As an example, if there is inadequate illumination during high-power magnification of the specimen 100, the image quality of the specimen 100 will be poor. As a result f this drawback, the lens device is unsuited for applications requiring a higher magnification power.
Reference is now made to FIGS. 3 and 4. There exists another lens device 2 that alleviates the aforementioned problems by incorporating a light emitting unit 211. lighting problems.
A general principle for obtaining an optimal image quality of the specimen 100 is to calibrate the lens device 2 with a proper light balance. That is, different magnification powers require different intensities of the light outputted by the light emitting unit 211. However, the intensity of the light provided by this lens device 2 is non-adjustable, and hence the lens device 2 is incapable of acquiring an optimal image quality of the specimen 100 for a wide range of magnification power.